Owing to their delicate embodiment, high quality requirements are often set for hollow valves, which must also be able to include samples up to a 100% testing. In particular, the maintaining of a predetermined wall thickness is an important quality factor in such hollow valves. In the simplest case in the case of a valve which is only bored hollow, this is the wall thickness in the stem and here, in particular, a possibly present centre offset of this bore, which has an effect on the wall thickness of the stem of the hollow valve. In this case, this can be established by measurement of the wall thickness on at least two or, better, four places or ideally even continuously on the circumference. In the case of constructed hollow valves made of sheet metal parts which are shaped and are welded to one another, the wall thickness measurement on the blank or respectively on the finished installed valve is likewise an essential quality feature, because through the shaping- and welding process deviations of the wall thickness can occur. In the case of hollow vales, on the other hand, in which a cavity is produced in the valve head by means of electrochemical machining (ECM), likewise the measurement of the wall thickness is extremely important, because electrochemical machining is poorly visible and hence poorly controllable and, moreover, is not defined by a solid cutter, such as for example in the production of a cavity with a drill. At the same time, in electrochemical machining a curved and irregular surface is produced, which in addition also forms an undercut. If a wall thickness can still be measured here tactilely in a relatively uncomplicated manner on the stem or respectively on the plate base, this is only possible with difficulty for the measurement of the wall thickness in the region of the chamfer. However, the measuring of the wall thickness in the region of the stem also already requires a comparatively long measurement time and a tactile approach. In addition, only the blank can be measured, because the finished produced valve generally does not enable any access for a tactile approach, as it has been filled with a cooling medium and closed. In order to save measuring time and in order to also be able to measure the wall thicknesses as simply as possible in the region of the chamfer, an ultrasonic measurement can be used, which, however, often presents difficulties in the case of curved surfaces and hence non-constant thicknesses or respectively oppositely-directed surfaces.
From DE 10 2007 011 445 B4 an ultrasonic method is known for the measurement of a wall thickness of a welded hollow beam, in which the hollow beam is closed at the ends in a pressure-tight manner by means of sealing elements and is acted upon by means of negative pressure or respectively positive pressure. Subsequently, a noise level is measured by an inspection unit, which is movable in the hollow beam, with an acoustic sensor directed substantially to the region of the weld seams, said noise level being produced by inflowing or outflowing air in the case of a crack in the weld seams of the hollow beam. To locate a crack which is detected by means of a flow noise, the respectively covered travelling distance of the acoustic sensor is determined by means of a path length measurement unit. This method is, however, able to be applied exclusively for comparatively large diameters of hollow sections.
From DE 202 07 684 U1 a device is known for ultrasonic thickness measurement within pipes, which is moved in the interior of a pipeline which is to be monitored and which has a housing with pieces of equipment contained therein for the measuring, processing and storing of measurement data. These pieces of equipment comprise an initiating pulse generator, an ultrasonic transmitter, an amplifier, a comparator with an analogue input, a digital clock timer, a processor and a digital data memory connected in series. This device is also, however, again moved in the interior of the pipeline which is to be measured, so that here, also, the cavity must have a certain diameter.